Chlamydia pneumoniae is an obligate intracellular bacteria first described in association with acute respiratory infections in 1985 that has recently been associated with human atherosclerotic cardiovascular disease. Chlamydia pneumoniae has been associated with atherosclerotic cardiovascular disease in seroepidemiologic studies, by detection of the organism in atherosclerotic plaque, and in animal model studies. The results of two small trials, which evaluated the use of azithromycin or roxithromycin among persons with documented coronary artery disease have indicated a possible effect of these agents in the secondary prevention of coronary heart disease outcomes. The evidence supporting this association provides a basis for initiation of further well designed and adequately powered clinical trials to assess the effect of treatment with antibiotics active against C. pneumoniae on the occurrence of cardiovascular disease outcomes. The study is a randomized, double blind multicenter trial of azithromycin vs. placebo among adults with documented prevalent coronary artery disease. Study participants will be treated for one year at a dose of 600 mg of azithromycin orally once per week and followed for a mean of 3.75 years for the composite primary outcome of CHD death, non-fatal myocardial infarction (MI), hospitalization for unstable angina, and requirement for coronary artery bypass grafting (CABG) or percutaneous revascularization procedures.